1. Field of the Invention
The present invention is related to combustion and heavy duty exhaust systems and methods for reducing nitrogen dioxide emissions.
2. Background Art
The control of NO2 emissions from combustion systems, such as diesel powered equipment, is becoming an increasingly important problem worldwide. This is at least partially a result of the implementation of a variety of exhaust purification devices which while reducing emission levels of particulate matter (“PM”), CO, and/or hydrocarbons (“HC”), simultaneously oxidize NO to NO2. The formation of NO2 necessitates NO2 suppression strategies for such systems.
Many of the prior art exhaust devices such as diesel oxidation catalysts (“DOC”), diesel particulate filters (“CDPF”) and their combinations utilize Pt because of its unique oxidation activity for the conversion (i.e., oxidation) of CO, HC, and PM under oxygen-rich conditions. Pt is also the most active of the precious metal catalysts for oxidation of NO to NO2. In certain prior art systems, NO2 generation is actually maximized to provide NO2 as an oxidant to facilitate the low temperature combustion of soot.
It is also known that NO2 can react with other species such as CO and HC. Catalytic studies indicate that because of this reactivity, NO2 does not begin to accumulate in significant quantities until CO and HC are largely eliminated from the reacting gas composition when passed across a precious metal catalyst. Pd is another well known catalyst for the oxidation of CO and HC and has been used broadly in aftertreatment catalysts for this purpose. Numerous studies have also shown that Pd is a poor catalyst for the oxidation of NO to NO2. The combination of these chemical reactivities results in exhaust systems with less than optimal NO2 emission characteristics.
Accordingly, there is a need for improved exhaust systems and methods for reducing the amounts of NO2 present in the exhaust of combustion systems.